I am actually partial to the Magic Sytem in Shodowrun.  It is based on Fatigue.  IE. every spell has a formula for how tired it makes you.  And their are modifiers for all kinds of stuff.  So you could cast the same spell twice in a row and have totally different levels of tired from it.  Like you get punched in between them your second one is going to affect you alot more than the first. 

The whole idea of fatigue seems alot more realistic than any other system.   You cast a powerful spell first, then all the other much simplier spells after it are still going to be alot harder because your already tired.